Origin of Uniparental Disomy 15 in Patients With Prader-Willi or Angelman Syndrome

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1 American Journal of Medical Genetics 94: (2000) Origin of Uniparental Disomy 15 in Patients With Prader-Willi or Angelman Syndrome Cintia Fridman* and Célia P. Koiffmann Department of Biology, Institute of Bioscience, University of Sâo Paulo, Sâo Paulo, Brazil Maternal uniparental disomy (UPD) accounts for 25% of Prader-Willi patients (PWS) and paternal UPD for about 2 5% of Angelman syndrome (AS) patients. These findings and the parental origin of deletions are evidence of genomic imprinting in the cause of PWS and AS. The natural occurrence of UPD individuals allows the study of meiotic mechanisms resulting in chromosomal nondisjunction (ND). We selected patients with UPD15 from our sample of 30 PWS and 40 AS patients to study the origin of ND and the recombination along chromosome 15. These patients were analyzed with 10 microsatellites throughout the entire chromosome 15 (D15S541, D15S542, D15S11, D15S113, GABRB3, CYP19, D15S117, D15S131, D15S984, D15S115). The analysis disclosed seven heterodisomic PWS cases originating by meiosis I (MI) ND (four showed recombination and three no recombination), and one isodisomic PWS UPD15 originating by postzygotic duplication. Among the five paternal UPD15, we detected four isodisomies, three of which showed homozigosity for all markers, corresponding to a mitotic error, and one case originating from a paternal MII ND. Our results indicate that besides maternal MI and MII ND, paternal ND occurs when a PWS UPD15 patient originates from mitotic duplication of the maternal chromosome 15. ND events in AS are mainly due to mitotic errors, but paternal MII ND can occur and give origin to an AS UPD15 individual by two different mechanisms: rescue of a trisomic fetus or fertilization of a nullisomic egg with the disomic sperm, and in this case paternal and maternal ND are necessary. Am. J. Med. Genet. 94: , Wiley-Liss, Inc. Grant sponsor: FAPESP; Grant number: C.F. 95/7161-0; Grant sponsor: PRONEX. *Corresponding author: C. Fridman, Departamento de Biologia, Instituto de Biociências, USP, Caixa Postal CEP: , São Paulo, SP, Brazil. cfridman@ib.usp.br Received 6 March 2000; Accepted 23 May Wiley-Liss, Inc. KEY WORDS: uniparental disomy; Angelman syndrome; Prader-Willi syndrome; nondisjunction, meiosis INTRODUCTION The differential expression of maternal and paternal genetic material has been referred to as genomic imprinting [Engel, 1980; Hall, 1990]. Prader-Willi syndrome (PWS; neonatal hypotonia, hyperphagia, severe obesity, short stature, and mental retardation with learning disabilities) and Angelman syndrome (AS; ataxia, seizures, sleep disorder, hyperactivity, severe mental retardation with lack of speech, and a happy disposition with paroxysms of laughter) are neurobehavioral disorders that result from the loss of expression of imprinted genes in the paternal and maternal chromosome 15, respectively. This can occur through different mechanisms such as deletion of 15q11-q13, UPD, imprinting mutation, and mutations in UBE3A in AS cases [Nicholls et al., 1998]. Maternal UPD accounts for 25% of PWS patients [Mascari et al., 1992; Robinson et al., 1991], and paternal UPD for about 2 5% of AS patients [Bottani et al., 1994; Malcolm et al., 1991; Nicholls et al., 1992]. These findings and the parental origin of deletions are evidences of genomic imprinting involvement in the PWS and AS etiology. The origin of UPD individuals depends primarily of nondisjunction (ND) events that can occur either in meiosis I (MI) or meiosis II (MII). There are at least three major mechanisms responsible for the formation of UPD15: (1) the origin of a trisomic fetus followed by the loss of one chromosome 15; (2) the fertilization of a disomic egg by a nullisomic sperm (gametic complementation); and (3) postzygotic duplication [Cassidy et al., 1992; Engel, 1993; Engel and DeLozier-Blanchet, 1991; Mascari et al., 1992; Mutirangura et al., 1993; Purvis-Smith et al., 1992; Robinson et al., 1993a, 1993b]. The difference between the incidence of PWS UPD and of AS UPD is attributed to the higher frequency of ND in female gametogenesis and, as for trisomy 21, it is correlated with increasing maternal age [Antonarakis et al., 1992; Robinson et al., 1996]. Chromosome pairing and recombination are impor-

2 250 Fridman and Koiffmann tant mechanisms that assure the correct segregation of the homologous chromosomes to the opposite poles at the end of MI [Carpenter, 1994]. Thus, failure in homologous pairing or reduction of the recombination can predispose chromosomes to ND [Robinson et al., 1993a]. Robinson et al. [1998] pointed out that there are at least two distinct mechanisms leading to ND: one related to achiasmate tetrads and one resulting in abnormal segregation of chromosome pairs with a normal level of recombination. Despite the fact that achiasmate pairs are the preferential target for ND, it is dependent on maternal age, since in young women achiasmate pairs can segregate normally, but they are more susceptible to ND in older women [Robinson et al., 1998]. Recombination analysis is interesting to determine if ND occurred in an achiasmate pair or due to an excess of chiasmata, resulting in delayed chromosome separation. In the present study we selected patients with UPD15 from our PWS and AS samples to study the origin of ND and the recombination throughout the entire chromosome 15, to contribute to the understanding of the meiotic and mitotic mechanisms responsible for the etiology of the PWS and AS. MATERIAL AND METHODS The UPD cases were obtained from a sample of 30 PWS and 40 AS patients diagnosed by methylation pattern studies of SNRPN exon 1 (data not shown). These UPD patients were investigated by analysis of 10 microsatellites throughout the entire chromosome 15 (D15S541 and D15S542 [Christian et al., 1995], D15S11, D15S113 and GABRB3 [Mutirangura et al., 1993], CYP19 [Polymeropoulos et al., 1991], D15S117, D15S131, D15S984 and D15S115 [Dib et al., 1996]) (Fig. 1). The origin of nondisjunction was established using the most centromeric markers (D15S541 and D15S542). The heterodisomic state of these markers indicates an MI error and their isodisomy indicates nondisjunction in the MII stage or a postzygotic event [Robinson et al., 1993a]. The other markers allowed us to disclose crossover regions (transition from hetero- to isodisomy and vice versa) in each case. The mitotic error was considered when all markers showed reduction to homozygosity. The genomic DNA (100 ng) was amplified in a polymerase chain reaction (PCR) of total volume of 10 l, with an initial denaturation for 4 min at 94 C. Thirty cycles were run, with denaturation for 30 sec at 94 C, annealing for 1 min at 55 C, and extension for 45 sec at 72 C. The final extension was for 6 min at 72 C. The microsatellites D15S11, GABRB3, and D15S113 were amplified by the method of Mutirangura et al. [1993] in a multiplex PCR. The PCR products were applied to a 4.8% polyacrilamide gel and electrophoresed for 3 hours followed by exposure to an X-ray film for 24 hours. RESULTS Out of a total of 30 PWS patients, 8 (23.4%) were found to have maternal UPD, 18 (53%) a paternal de- Fig. 1. Physical and genetic map of the markers used in UPD study. The order of the markers and the distances (cm) are based on Dib et al. [1996] and Robinson and Knoll [1997].

3 UPD in PWS and AS 251 letion of 15q11-q13, and 4 were uninformative regarding the genetic mechanism. Among 40 AS patients, 5 (12.5%) presented paternal UPD, 24 (60%) showed maternal deletion of 15q11-q13, 3 were uninformative, and 8 were normal for all genetic tests but continued to have a clinical diagnosis of AS. By analyzing 10 microsatellite loci (CA repeats) spanning 15q we identified the meiotic origin of ND in all cases of UPD but two (patient 4, PWS; and patient 11, AS), and also the number of transitions ( observed changes in marker state between heterodisomy and isodisomy) in each case. Although in patient 4 (PWS) it was not possible to establish the meiotic origin of ND, we considered this case as a heterodisomy since the D15S11 locus is mapped only 3cM from the centromere (Fig. 1). In patient 11 (AS) it was not possible to disclose the origin of ND because there was no paternal sample for analysis and only two loci were informative within the PWS/AS region. The results were interpreted according to Robinson et al. [1993a]. Thus, we disclosed seven PWS heterodisomies originating from maternal MI ND (three showed heterodisomy throughout the entire chromosome with no evidence of recombination [patients 4, 5, and 6]) and four with some loci showing hetero- and others isodisomy (patients 1, 3, 7, and 8)], 1 AS isodisomy (MII paternal error, patient 10), four isodisomies resulting from mitotic errors (1 PWS [patient 2] and 3 AS [patients 9, 12, and 13]). AS patient 9 was really an isodisomy due to a translocation 15;15 that was previously described [Fridman et al., 1998]. The microsatellite results of PWS and AS patients are shown in Table I and drawn in Figure 2. AS patients 9, 10, 11, and 12 were already described (Fridman et al., 2000b) corresponding to patients 1, 2, 3, and 4, respectively. Maternal age was increased in our UPD sample. DISCUSSION In our UPD sample we detected eight maternal UPD corresponding to 23.4% of the PWS group and five paternal UPD, corresponding to 12.5% of the AS group. The maternal UPD frequency is in accordance with literature data [Nicholls et al., 1989; Robinson et al., 1991], but we found an increase of paternal UPD in our sample compared with others (2 3%) [Clayton-Smith and Pembrey, 1992; Malcolm et al., 1991]. This result could be explained by the fact that we did not use only the diagnostic criteria suggested by Williams et al. [1995] to consider a patient as a probable AS. We tested patients with absence crises, seizures of late onset, and obesity associated with outer frontal circumference TABLE I. Results of Microsatellite Analysis of UPD15 Patients and Their Parental Ages* D15S541 D15S542 D15S11 D15S113 GABRB3 CYP19 D15S117 D15S131 D15S984 D15S115 Maternal age (years) Paternal age (years) M 1,2 2,2 1,2 2,3 3,3 2,4 1,4 1,2 1,3 1,2 1 PWS 1,2 NR 2,2 1,2 NR 2,3 NR 3,3 2,4 NR 1,1 R 2,2 R 1,1 R 1,2 NR F 1,2 1,2 1,2 1,1 1,2 1,3 2,3 1,3 2,2 3,4 M 1,3 1,1 1,2 2,2 2,3 2,2 1,2 1,4 2,2 2,3 2 PWS 1,1 R 1,1 2,2 R 2,2 2,2 R 2,2 1,1 R 1,1 R 2,2 3,3 R F 2,3 2,2 1,3 1,1 1,2 1,1 1,2 2,3 1,3 1,1 M 1,2 3,4 1,1 1,3 1,2 1,2 1,4 2,3 1,2 3,3 3 PWS 1,2 NR 3,4 NR 1,1 1,3 NR 1,2 NR 2,2 R 4,4 R 2,2 R 1,1 R 3, F 1,1 1,2 1,2 2,2 3,4 1,2 2,3 1,1 3,4 1,2 M 2,2 3,3 1,2 1,2 1,2 1,2 1,3 2,2 2,3 1,3 4 PWS 2,2 3,3 1,2 NR 1,2 NR 1,2 NR 1,2 NR 1,3 NR 2,2 2,3 NR 1,3 NR F 1,1 1,2 2,2 1,3 2,3 1,1 2,3 1,4 1,4 1,2 M 1,2 1,3 1,2 1,2 2,3 1,1 1,4 1,2 5 PWS 1,2 NR 1,3 NR 1,2 NR 1,2 NR 2,3 NR 1,1 2,3 NR 1,4 NR 1,2 NR F 1,1 2,2 1,2 1,2 1,1 2,3 1,2 2,3 1,3 M 1,1 1,2 2,3 3,3 1,1 1,3 2,3 2,3 1,3 2,3 6 PWS 1,1 1,2 NR 2,3 NR 3,3 1,1 1,3 NR 2,3 NR 2,3 NR 1,3 NR 2,3 NR F 2,2 3,3 1,4 1,2 1,2 1,1 1,4 1,4 2,4 1,4 M 1,2 1,2 1,1 1,2 1,2 2,3 2,3 3,4 2,3 1,2 7 PWS 1,2 NR 1,2 NR 1,1 1,2 NR 1,2 NR 2,3 NR 3,3 R 3,4 NR 3,3 R 1,2 NR F 1,1 1,2 2,2 2,2 1,1 1,3 1,4 1,2 1,2 1,2 M 1,1 1,2 1,3 1,3 1,2 1,2 2,3 1,3 2,4 1,3 8 PWS 1,1 1,2 NR 1,3 NR 1,3 NR 1,2 NR 2,2 R 3,3 R 1,3 NR 2,4 NR 1,3 NR F 1,2 1,1 2,4 1,2 3,3 1,2 1,4 2,3 1,3 2,3 M 3,4 3,4 2,4 1,1 1,2 1,4 2,2 3,3 2,3 1,2 9 AS* 1,1 R 1,1 R 1,1 R 1,1 R 3,3 R 2,2 R 3,3 R 3,3 R 2,2 R 4,4 R F 1,2 1,2 1,3 1,2 3,3 2,3 1,3 1,3 1,2 3,4 M 2,3 2,3 1,2 2,3 3,3 1,1 2,3 1,3 2,3 2,2 10 AS* 1,1 4,4 R 1,1 R 2,2 R 1,1 R 1,2 NR 1,4 NR 2,4 NR 1,1 1,3 NR F 1,1 1,4 1,3 1,2 1,2 1,2 1,4 2,4 1,1 1,3 M 1,1 1,1 1,4 2,3 1,3 1,2 1,2 1,3 1,2 11 AS* 2,2 2,2 2,3 NR 1,2 NR 2,2 1,1 1,1 2,2 2, F M 1,3 1,3 2,2 1,1 2,4 1,2 1,2 1,3 1,3 12 AS* 2,2 R 2,2 R 1,1 R 2,2 R 1,1 R 4,4 R 2,2 R 3,3 R 4,4 R F 2,4 2,3 1,3 2,3 1,3 3,4 2,3 2,3 2,4 M 2,3 1,2 3,3 1,1 3,4 1,1 1,2 1,2 3,4 13 AS 1,1 3,3 R 1,1 R 1,1 2,2 R 1,1 3,3 R 4,4 R 1,1 R F 1,1 3,4 1,2 1,1 1,2 1,2 3,4 3,4 1,2 *Results already presented in Fridman et al. [2000b]. R, reduction; NR, non reduction; M, mother; F, father;, not tested.

4 252 Fridman and Koiffmann Fig. 2. Schematic results of UPD PWS (A) and AS (B) patients and the localization of the markers along chromosome 15. HET, heterodisomy; ISO, isodisomy; NI, noninformative; NT, not tested; NR, nonreduction; R, reduction. (OFC) above the 75th centile. In this way, we included cases that probably are borderline for the still incompletely known phenotypic spectrum of AS [Fridman et al., 1998; Fridman et al., 2000b]. As reported previously [Mascari et al., 1992; Mutirangura et al., 1993; Robinson et al., 1993a, 1996] we also detected a higher incidence of maternal ND at MI in PWS UPD15 patients and mitotic errors in AS UPD15 cases. These results confirm that most maternal ND events resulting in UPD15 PWS are associated with MI errors, with rare cases being MII or due to postzygotic errors, whereas most paternal UPD15 seems to be postzygotic events. Recombination was detected in four maternal and one paternal UPD15 (patient 10; Fridman et al., 2000a) (Fig. 2). Robinson et al. [1993a] suggested that the use of markers with intervals of 20 cm is sufficient to observe most crossover events. Although some markers used here were mapped less than 20 cm from each other, we could observe recombination, as shown in patients 1 and 7, where there is a transition from heterodisomy (nonreduction) to isodisomy (reduction) between the loci CYP19 and D15S117, localized 8 cm from each other, and transition from reduction to nonreduction between the loci D15S984 and D15S115, mapped 7 cm from each other (Table I; Fig. 2). In three maternal UPD15 cases we cannot conclude that there was no meiotic recombination since both homologous chromatids that segregated together could be the two involved in the crossover, and then the recombination event would not be observed. These cases can also represent achiasmatic pairs, and so there are no crossover points. In our UPD PWS sample we also observed an increase in maternal age (Table I) confirming that, also for chromosome 15, there is a correlation between increased maternal age and nondisjunction. It is interesting to note that in postzygotic errors resulting in PWS individuals with UPD15 (such as patient 2) the primary event is a paternal rather than a

5 UPD in PWS and AS 253 maternal ND, since the fertilization of a normal egg with a nullisomic sperm followed by a mitotic duplication of maternal chromosome 15 is necessary. In AS the postzygotic errors are associated with increased maternal age since the primary event is the maternal ND followed by the fertilization of the nullisomic egg with a normal sperm and later duplication of the paternal chromosome in the zygote. The origin of an AS patient with isodisomy due to MII error [Fridman et al., 2000a] can be attributed to two mechanisms: paternal ND in MII and fertilization of a normal egg with this disomic sperm originating a trisomic fetus with the later loss of the maternal chromosome; or the fertilization of a disomic sperm with a nullisomic egg, for which a maternal and a paternal ND is necessary. In conclusion, in the sample of eight PWS UPD15 cases we detected seven with heterodisomy originated by MI ND, of which four showed recombination and three no recombination along chromosome 15. One PWS UPD15 was an isodisomy that was probably originated by an event of mitotic duplication, rare in PWS. In the sample of five paternal UPD15, we detected four isodisomies, three of which showed homozygosity for all markers, corresponding to a mitotic error or, alternatively, to a MII meiotic error where no recombination has occurred in MI, and one case was originated by a paternal MII ND since it showed a recombination point. The fifth case was uninformative regarding the meiotic origin of ND. The natural occurrence of UPD individuals allows the study of the meiotic mechanisms resulting in chromosomal ND and the understanding of the influence of parental age on this process. 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